This adds a new subcommand:
inspec env [SHELL]
which outputs a shell-appropriate completion script that the user can
source into their shell:
eval "$(inspec env SHELL)"
Currently, we provide completions for ZSH and Bash. The completion
scripts are generated from the data Thor collects.
If the user doesn't provide SHELL we attempt to detect what the user's
shell may be using a number of methods.
Signed-off-by: Steven Danna <steve@chef.io>
